Cu(410): Interaction and Adsorption Geometry

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

CHCl/Cu(410): Interaction and Adsorption Geometry Takamasa Makino, Siti Zulaehah, Jessiel Siaron Gueriba, Wilson Agerico Diño, and Michio Okada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01296 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

CH3Cl/Cu(410): Interaction and Adsorption Geometry Takamasa Makino,† Siti Zulaehah,‡ Jessiel Siaron Gueriba,‡ Wilson Agerico Diño,∗,‡,¶ and Michio Okada∗,† †Institute for Radiation Sciences and Department of Chemistry, Osaka University, Toyonaka, Osaka 560-0043, Japan. ‡Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan. ¶Center for Atomic and Molecular Technologies, Osaka University, Suita, Osaka 565-0871, Japan. E-mail: [email protected]; [email protected]

Abstract The Rochow process is the most common technology used to prepare organosilicon compounds on an industrial scale, and yet the mechanism is still not well understood. It involves the reaction of methyl chloride (CH3Cl) with silicon, catalyzed by copper. To understand the elementary steps of the reaction involved, we studied the molecular adsorption of CH3Cl/Cu(410) at 100 K, and its complete desorption at higher temperatures, 100 K < TD < 200 K. Temperature-programmed desorption (TPD) spectra show two CH3Cl desorption peaks. We attribute the low temperature TPD peak (TD ∼ 120 K) to CH3Cl desorbing from both step-edges and terraces, and the high temperature TPD peak (TD ∼ 160 K) to CH3Cl desorbing from the stepedges. Infrared reflection-absorption spectra (IRAS) indicate that at low CH3Cl coverage (Θ = 0.06 ML), CH3Cl adsorbs with its molecular axis (Cl-C bond) aligned either

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parallel or perpendicular to [001]. At high CH3Cl coverage (Θ ≥ 0.09 ML), CH3Cl adsorbs with its molecular axis aligned perpendicular to [001].

Introduction The direct synthesis of organosilanes, a.k.a. the Rochow reaction, 1 has been intensively used in industrial plants all over the world, e.g., for the production of silicone. 2,3 Fundamental studies on various model systems 4–6 have been done trying to unravel the mechanism behind the corresponding associated reactions with varying degrees of success. In the Rochow reaction, Cu catalyzes the (direct) reaction: Si + 2CH3Cl → (CH3)2SiCl2. As one would expect, the (direct) reaction of CH3Cl with pure Si plays an important role in understanding the reaction mechanism. Surface science studies 7–19 found dissociative adsorption of CH3Cl/Si(100) and molecular adsorption of CH3Cl/Si(111). On the other hand, contrary to expectations, studies have found Cu3Si alloy surfaces rather inert to CH3Cl. 20,21 It is still not clear how Cu catalyzes the (direct) reaction. Experimental studies report molecular chemisorption (no thermal decomposition) of CH3Cl/Cu(110), 22 and that CH3Cl bonds to Cu(110) through Cl. Studies on other metal surfaces, e.g., Pt(111), 23,24 Ag(111), 25 Pd(100), 26 and Ru(001), 27 report similar results. Infrared reflection-absorption spectroscopy (IRAS) on Pt indicates that the CH3Cl adsorption geometry depends on the CH3Cl coverage Θ, i.e., lying-down (Cl-C bond oriented parallel to the surface) at low and straight-up (Cl-C bond oriented perpendicular to the surface) at high Θ. 24 Other studies report multilayer desorption. 24–28 On Ni(100), 28 temperatureprogrammed desorption (TPD) measurements suggest CH3Cl decomposition, although in small amounts (less than 0.01 ML of decomposed CH3Cl). Auger electron spectroscopy (AES) 28 performed after TPD measurements did not detect any surface C nor Cl atoms. Catalysts surfaces consist of atomic-scale steps, terraces, and other defects that play an important role in the corresponding chemical reactions, such as the Rochow reaction.

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To obtain a fundamental understanding of the effect of steps, we need a relatively stable surface. The Cu(410) provides us with a stable surface for adsorption to study the role of the step-edge atoms. As such, we have previously studied the adsorption of C2H4, 29 CO, 30 and O2 31 on Cu(410). Similarly, we expect to gain further understanding regarding the Rochow reaction considering the adsorption of CH3Cl/Cu(410). Here, we report a detailed study on the adsorption of CH3Cl/Cu(410) by performing TPD and IRAS experiments, together with density functional theory (DFT)-based total energy calculations. We found that CH3Cl molecularly adsorbs on Cu(410) at 100 K. The molecularly adsorbed CH3Cl completely desorbs at higher temperatures, 100 K < TD < 200 K. The TPD spectra show two CH3Cl desorption peaks. We attribute the low temperature TPD peak (TD ∼ 120 K) to CH3Cl desorbing from both the terraces and the step-edges, and the high temperature TPD peak (TD ∼ 160 K) to CH3Cl desorbing from the step-edge. The IRAS spectra and total energy calculations indicate chemisorption geometry that varies with CH3Cl coverage Θ. At low CH3Cl coverage (Θ = 0.06 ML), CH3Cl adsorbs with its Cl-C bond oriented either parallel or perpendicular to [001]. At high CH3Cl coverage (Θ = 0.09 ML), CH3Cl adsorbed with its Cl-C bond oriented perpendicular to [001].

Experimental Methodology Set-up: We performed the experiments in an ultra-high-vacuum (UHV) chamber pumped down to a base pressure of < 4 × 10−8 Pa, using a combination of turbo-molecular pump, titaniumsublimation pump, and a nitrogen trap. We can perform Fourier-transform infrared (FTIR) spectroscopy, quadrupole mass spectroscopy (QMS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) analyses within the same UHV chamber. One can find a schematic diagram of the apparatus in Ref. 32.

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Clean Cu(410) sample preparation and characterization: We used mechanically polished single crystal surfaces (15 mm ×15 mm×1.5 mm, Surface Preparation Laboratory). To prepare the clean Cu(410) sample, we bombarded the surface with Ne ions (2 keV, ∼ 3µA·cm−2 ) for 30 min, and subsequently anneal to above 670 K for 20 min. We performed this cleaning process repeatedly, and monitored the sample cleanliness by AES and LEED. We terminate the cleaning process once when we observed a sharp LEED pattern corresponding to Cu(410) (indicating no surface reconstruction). Ideally, Cu(410) has three atomic widths of (100) terraces and (110) steps, and [100] subtends an angle of 14.0◦ with respect to [410] (the surface normal).

Vibrational spectroscopy: We do IRAS using an FTIR spectrometer (JASCO, FTIR 6100) with a liquid-nitrogen cooled HgCdTe (MCT) detector. We use a wire grid polarizer to prepare p-polarized incident IR beam, which we then focused through a BaF2 window onto the sample surface at an incident angle of 80◦ off-normal, with an average incident azimuth angle of 15◦ off the step-edge direction. We use a dried air generator (Parker Balston, 7562JA) to supply carbon dioxidefree dry air to the optical paths and the interferometer. We have a spectral resolution of 4 cm−1 and we typically collect 512 scans for the present experiments. The spectral range lies between 800 cm−1 and 3500 cm−1 , although stable operation would be difficult below 950 cm−1 .

Thermal desorption spectroscopy: We carry out TPD measurements using an QMS enclosed by a random-flux shield. To maximize the desorbing molecule signal during TPD, we positioned the QMS sampling orifice ca. 1 mm from the sample. We use liquid nitrogen dewar to cool down the sample. To carry out the TPD measurements, we use a heating rate of 2.3 K/s, and monitored the signal

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at m/e = 50 (CH3Cl+ ) using QMS. For gas exposure, we backfill the chamber with high purity CH3Cl (Matheson), and measure the gas exposure in units of Langmuir (1.0 L= 1.33 × 10−4 Pa·s), without any sensitivity corrections.

Results and Discussion TPD spectra of CH3Cl/Cu(410): In Fig. 1(a), we show how the TPD spectra from CH3Cl/Cu(410) (at 100 K) change as we increase the CH3Cl gas exposure from 0.1 L to 5.0 L. At 0.1 L exposure, we observe a slightly asymmetric β peak (TD = 169 K). As we increase the CH3Cl gas exposure from 0.1 L to 2.0 L, we see a gradual shift in the β peak position from TD = 169 [email protected] L to TD = 158 [email protected] L. The asymmetric shape and slight peak shifts indicate first-order desorption kinetics. 33 After the β peak saturates, a new α peak appears and increases in intensity with further increases in exposure (from 2.0 L to 5.0 L). In Fig. 1(b), we show the corresponding uptake curve determined from the TPD spectra in Fig. 1(a). Considering natural abundance, we counted both CH337 Cl and CH335 Cl to determine the uptake curve. (Note: 1 monolayer [ML] corresponds to 1.49 × 1015 molecules/cm2 on Cu(410). For more details on how we determined the coverage Θ[ML], cf., Supplementary Material.) The uptake increases linearly with exposure, for exposures less than 2.0 L. AES spectra show no increase in carbon KLL and Cl LMM peaks, after CH3Cl desorption.

β-peak of CH3Cl/Cu(410): From the β-peak of the TPD spectra, we can determine the activation energy Edes (Θ) for CH3Cl desorption from Cu(410), using the inversion-optimization method. 34 Each TPD spectra gives the desorption rate r(Θ, T ) for a given initial (instantaneous) coverage Θ and heating rate. We adopt the Polanyi-Wigner rate expression to describe the thermal desorption of CH3Cl in terms of the pre-exponential factor νn , kinetic order n, and the universal gas 5



Figure 1: (a) TPD spectra taken at a heating rate of 2.3 K/s, for CH3Cl adsorbed on Cu(410) at 100 K, after CH3Cl exposures ranging from 0.1 L to 5.0 L. α peak: low temperature TPD peak at TD ∼ 120 K. β peak: high temperature TPD peak at TD ∼ 160 K. (b) Uptake curve for CH3Cl determined from the TPD peak area. The straight line is a least square fit to the data from 0.1 L to 2.0 L exposure. The dashed line extending further corresponds to saturation of the β peak.

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constant R. We assume first-order desorption (n=1) and that the pre-exponential factor νn = ν1 = ν does not depend on either Θ or T , within the range of the β peak. Upon mathematical inversion of the corresponding Polanyi-Wigner rate expression, we get [

] [ ] dΘ/dt r(Θ, T ) Edes (Θ) = −RT ln = −RT ln − . νΘ νΘ

(1)

For each known coverage Θ, temperature T , and pre-factor ν, we can then determine Edes (Θ) from the desorption rate (the TPD signal). We calculated Edes (Θ) using Eq. (1) and the corresponding TPD spectrum for 0.7 L exposure, assuming various pre-factor ν values. We assumed that the Edes vs. Θ curves would exhibit the same trend for a correctly chosen pre-factor ν, regardless of the selected TPD spectra. Using the determined Edes vs. Θ curves for various assumed pre-factor ν, we can then simulate the TPD spectra for lower exposures, viz., 0.3 L and 0.5 L. We evaluated the best-fitted ν by calculating a χ2 error between the simulated curves and experimental TPD curves. In Fig. 2(a), we replot the experimentally determined β peak and the corresponding simulated curves using the best-fit value ν = 1013.7±1.4 s−1 . In Fig. 2(b), we plot the coverage dependent desorption energy Edes (Θ) curve for the optimized pre-factor ν determined from the above-mentioned inversion analysis. We represent the coverage-dependent desorption energy Edes (Θ) for Θ(ML): [0.0, 0.072] using the following analytical expression: ( ) Θ Edes (Θ) = E0 + γΘ + Edef exp − . Θdef

(2)

This expression captures the general behavior of the desorption energy with respect to coverage without unnecessary complexity. We obtained the first term E0 = 46.7 kJ/mol by extrapolating the linear region of Edes (Θ) vs. Θ curve to the zero-coverage limit, i.e., Edes (Θ = 0). The exponential pre-factor Edef = 4.30 kJ/mol comes from the difference between the extrapolated E0 and the measured Edes (Θ = 0). The denominator in the 7



Figure 2: (a) Square and Circles: TPD spectra (β peak) for CH3Cl/Cu(410) @100 K. Lines: simulated results.(b) Simulated [lines, Eq. (3)] and experimental (circles) desorption energy as a function of coverage.

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exponential argument Θdef = 1.10 × 10−3 ML gives a measure of the rate at which the influence of defect sites on the energy result decays with increasing Θ. We can thus rewrite Eq. (2) as follows: ( Edes (Θ) = 46.7 − 30.0Θ + 4.30 exp −

Θ ) . 0.00110

(3)

Edes = 46 ± 5 kJ/mol at Θ = 0.036 ML. Applying the simple Redhead formula 35 with a pre-exponential factor of ν ∼ 1013 s−1 , one would obtain a value Edes = 43 kJ/mol for the β state. For comparison, Edes = 42 kJ/mol for CH3Cl/Cu(110) at low coverage, 22 and Edes = 46.9 kJ/mol for CH3Cl/Pd(100) on the defect sites. 26

Figure 3: Desorption energy as a function of coverage. Energies are calculated from the TPD spectra at 0.53 ML coverage, assuming the pre-factor ν = 1013.7±1.4 s−1 obtained for the β peak.

Using the same method of analysis for the α-state CH3Cl/Cu(410), we could not get a 9



good fit to the TPD spectra. This is probably because the α peak consists of two components/adsorbate states (vide infra). Assuming ν = 1013.7 s−1 obtained for the β peak, we show the resulting Edes (Θ) curve determined from the TPD spectra at Θ = 0.53 ML in Fig. 3. Edes = 37.5 kJ/mol at Θ = 0.25 ML and Edes = 32.7 kJ/mol at 0.50 ML. One would also obtain Edes = 30 kJ/mol for the α state, applying the simple first-order Redhead formula 35 with a pre-exponential factor ν = 1013 s−1 . Again, for comparison, Edes = 28.2 kJ/mol for the desorption of condensed phase CH3Cl/Ag(111), 25 and Edes = 28.0 kJ/mol for CH3Cl/Pd(100). 26 Thus, we cannot ignore the possibility of multi-layer adsorption at high coverage.

IRAS SPECTRA OF CH3Cl/Cu(410):

Figure 4: IRAS spectra of adsorbed CH3Cl on Cu(410) at 100 K, and after varying exposures.

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Table 1: Experimental vibrational frequencies [cm−1 ] of gas phase and adsorbed CH3Cl. [ν/ρ/δ](a)s : (a)symmetric [stretching/rocking/bending] modes. Cu(410) Cu(110) 100 K, 5 L 100 K, 5 L (this work) HREELS 22 ν(M-Cl) ν(C-Cl) ρ(CH3) δs (CH3) δa (CH3) νs (CH3) νa (CH3)

1013 1336 1436 2925, 2954

696 1009 1390 1490 3000 3070

Pt(111) 105 K Pt(111) 100 K Pt(111) 100 K monolayer saturation low coverage monolayer saturation HREELS 23 IRAS 24 IRAS 24 not resolved 695 1040 1007 not resolved 1332 1435 1424 3020 2955

Pt(111) 100 K Gas phase multilayer (CH3Cl) IRAS 24 (Ref. 36)

1022 1336, 1347 1438, 1444 2953

732 1015 1355 1455 2966 3042

In Fig. 4, we show the corresponding IRAS spectra for CH3Cl adsorbed on Cu(410) at 100 K, after CH3Cl exposures ranging from 0.5 L to 5.0 L. To determine the correspondence between the IR bands and the CH3Cl vibrational modes, we follow previous reports, 22–24,36 (cf., Figs. 4 and Table 1). At 0.5 L (0.06 ML), we observe C-H symmetric stretching (νs (CH3)), with an anti-absorption component (@ 2916 cm−1 , cf., enlarged spectra in Fig. 5). At 0.7 L (0.09 ML), we observe C-H asymmetric bending (δas (CH3)) and C-H symmetric stretching modes appearing at 1426 and 2929 cm−1 , respectively. The C-H symmetric stretching mode has no anti-absorption component. At 1.0 L (0.12 ML), in addition to the C-H asymmetric bending and C-H symmetric stretching modes, we observe another peak corresponding to CH3 rocking (ρ(CH3)) mode at 1012 cm−1 . An additional C-H symmetric stretching mode appeared at 2957 cm−1 after 4.0 L (0.36 ML) exposure. In Table 1, we show a summary of our peak assignments and results from previous studies. From the IRAS results, cf., Figs. 4, 5 and Table 1, we see the close proximity between the vibrational modes of CH3Cl/Cu(410) and gas phase CH3Cl, indicating little perturbation by the surface. Thus, as previously observed on Cu(110), 22 CH3Cl also molecularly adsorbs on Cu(410). In Fig. 5, we show the IRAS peak corresponding to/assigned to the C-H symmetric stretching for CH3Cl/Cu(410) after 0.5 L (0.06 ML) exposure. We assumed that this peak (the change in IR reflectance, ∆R) has two components, viz., a normal IR absorption component and an anti-absorption component. We usually describe the former by a Lorentzian

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Figure 5: Circles: IRAS spectra in the region of C-H symmetric stretching for the exposure of 0.5 L. The fits to the experimental data consist of the following - dotted-line: background component, dashed-line: Lorentzian component, dash-dot lines: anti-absorption component, and solid lines: sum of all these components.

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function and the latter by the following equation:

∆Ranti = −

4M na (Ω2 − ω 2 )2 , mncτ cos θ (Ω2 − ω 2 )2 + (ω/τ )2

(4)

given in terms of the frequency ω of the oscillating electric field, the mass M of the adsorbate, the number of adsorbates per unit area na , the mass m of the electron, the electron density n (number of conduction electrons per unit volume), the speed of light c, the damping rate 1/τ due to excitation of electron-hole pairs, the incidence angle θ of the IR light, and the resonance frequency Ω. 37 In Fig. 5, we show the fitted results. We got a good fit by using the fitting parameter τ = 8.4 × 10−12 s (cf., Supplementary Material for more details), which is smaller than that for the parallel frustrated rotations of CO/Cu(100), i.e., τ = 2.6 × 10−11 s. 37,38 This may be caused by the step-edge adsorption. Anti-absorption line shapes in the IRAS spectra have been observed in studies on frustrated rotations of CO/Cu(100) and frustrated translations of H/W(100) and H/Mo(100). 38,39 These vibrational modes are usually dipole forbidden. 37 In the present study, observation of an anti-absorption line shape in the C-H symmetric stretching indicates a C-H bond oriented almost parallel to Cu(410). At 0.5 L (0.06 ML) exposure, we observe both antiabsorption and normal-absorption components in the IRAS spectra. This indicates two different CH3Cl configurations present on Cu(410) at low coverage. With increasing CH3Cl exposure, at least above 0.7 L (0.09 ML), the anti-absorption peak shape disappears in the C-H symmetric stretching. The disappearance of the anti-absorption peak indicates that all the remaining CH3Cl adsorbed with the same configuration on Cu(410). Similar coverage dependent chemisorption geometry was observed for CH3Cl/Pt(111), although the C-H symmetric stretching peak was not observed at a low coverage. 24 By taking the corresponding TPD and IRAS spectra at varying CH3Cl exposures, we can assign the TPD peaks to corresponding CH3Cl adsorption states at higher coverages. Saturation of the β peak corresponds to the step-edge adsorption of CH3Cl. As mentioned

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earlier, the α peak consists of two components/adsorbed species (terrace and step-edge) based on observed changes in the corresponding IRAS spectra at low and high coverages. At low coverages, which corresponds to exposures less than 3.0 L (0.27 ML), we see no changes in the IRAS spectra from those corresponding to the β peak saturation. Thus, we attribute the low coverage α peak component to CH3Cl desorption from the step-edge. On the other hand, at high coverages, which corresponds to exposures greater than 4.0 L (0.36 ML), we see an additional C-H symmetric stretching peak appearing at 2957 cm−1 , close to the gas-phase CH3Cl value. 36 Thus, we attribute the high coverage α peak component to CH3Cl desorption from the terrace sites. The changes in the corresponding IRAS spectra at low and high coverages suggest that the CH3Cl completely saturates the steps first, before starting to occupy the terrace. The IRAS spectra also indicate almost no effect on CH3Cl adsorbed on the step from CH3Cl adsorbed on the terrace. Table 2: Calculated results for gas phase and adsorbed CH3Cl. [ν/ρ/δ](a)s : (a)symmetric [stretching/rocking/bending] modes in units of [cm−1 ]. Eads : adsorption energy in units of [kJ/mol] and [eV]. rC−Cl(H) : C-Cl(H) bond-length in units of [Å].

Gas phase (CH3Cl) (Ref. 36) ν(M-Cl)[cm−1 ] ν(C-Cl)[cm−1 ] ρ(CH3)[cm−1 ] δs (CH3)[cm−1 ] δa (CH3)[cm−1 ] νs (CH3)[cm−1 ] νa (CH3)[cm−1 ] Eads [kJ/mol] Eads [eV] rC−Cl [Å] rC−H [Å]

Computation (this work) Θ = 1/16 ML Θ = 1/8 ML Gas phase (∼ 0.5 L) (∼ 1 L) (CH3Cl) Sa Sb Sa Sb

732 1015 1355 1455 2966 3042

718 1004 1334 1439 3018 3108

1.776 1.084

1.783 1.094

678 682 1003 1004 1330 1332 1424 1425 3008 3017 3099 3101 -23.54 -24.70 -0.241 -0.258 1.797 1.799 1.094 1.094

678 678 1000 1002 1325 1329 1425 1421 3007 3008 3093 3093 -19.58 -22.67 -0.205 -0.241 1.798 1.799 1.094 1.094

To further verify the adsorption states, we performed density functional theory (DFT)based total energy calculations, 40,41 using projector augmented wave (PAW) formalism, 42 14


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Figure 6: (a) Cu(410) surface with the various surface atoms/adsorption sites indicated, viz., step-edge atom (S, dark spheres) and terraces atoms (T1 , T2 , T3 , light spheres). (b) Sa configuration of CH3Cl/Cu(410) at low coverage at (1/16 ML). (c) Sb configuration of CH3Cl/Cu(410) at low coverage at (1/16 ML).

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with Perdew-Burke-Enzerhoff (PBE) generalized gradient (GGA) exchange 43,44 correlation functional, and a cutoff energy of 550 eV. We adopt the Monkhorst and Pack method 45 to perform the Brillouin zone integrations. The ground-state geometries of the bulk and surfaces are obtained by requiring an energy convergence criterion below 10−5 eV, and that the maximum force acting on each atom be below 0.01 eV/Å. The calculated optimized bulk lattice parameters for Cu a0 = 3.634 Å (cf., Ref. 46–48). To model Cu(410), we use a periodic slab of 16 Cu atomic layers, separated by 20 Å of vacuum. To simulate 1/8 ML and 1/16 ML CH3Cl coverages, we used (2 × 1) and (4 × 1) surface unit cells with (4 × 4 × 1) and (4 × 2 × 1) k-point meshes, respectively. From the calculated energies of the isolated CH3Cl ECH3Cl , the isolated Cu(410) ECu(410) , and the adsorbate system CH3Cl/Cu(410) ECH3Cl/Cu(410) , we determine the corresponding adsorption energies Eads = ECH3Cl/Cu(410) −(ECH3Cl +ECu(410) ) at four different sites, viz., step-edge atom S and terrace atoms T1 , T2 , T3 (cf., Fig. 6, Table 2). We found two possible molecular orientations, viz., with the Cl-C bond axis parallel (Sa) and perpendicular (Sb) to [001] (cf., Fig. 6). Total energies at different adsorption sites were calculated for both high (1/8 ML) and low (1/16 ML) coverages. Results show that CH3Cl prefer to adsorb on top of the step-edge atoms, with the Cl-C bond axis oriented nearly parallel to the surface. The calculated binding energies of CH3Cl on the step-edge site at high and low coverages differ by ∼ 1.93 kJ/mol (∼ 0.02 eV), corresponding to the temperature difference in the adsorption peaks between 1/8 ML and 1/16 ML, as shown in the TPD spectra. To determine the CH3Cl vibrational frequencies (Table 2), we adopted the harmonic approximation and used a displacement step-size of 0.01 Å in all direction. The calculated vibrational frequencies can be compared with the experimentally-determined ones (cf., Tables 1 and 2). From Table 2, we may infer that at low coverages (1/16 ML), the Sa and Sb configurations coexists, and at higher coverages (1/8 ML), the Sb configuration dominates/remains. In the Sa configuration (Fig. 6(b)), we find C-H bonds aligned parallel to the surface. We may attribute the detected anti-absorption peak component in the C-H stretching in the IRAS

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spectra to the presence of the Sa configuration at low coverage. We may attribute the disappearance of the anti-absorption peak component in the C-H stretching in the IRAS spectra to the absence of the Sa configuration and/or dominance of the Sb configuration at high coverage. Most simple adsorbates on metals interact repulsively, so that the average binding energies decrease with increasing coverage. For the case of CH3Cl, the decrease in binding energy with increasing coverage can be attributed to steric effects/steric hindrance. Also, due to steric effects/steric hindrance, the Sb configuration allows for more CH3Cl adsorption on the surface, as compared to the Sa configuration. (The trend in the calculation result of CH3Cl/Cu(410) adsorption does not change when we considered van der waals correction (vdW-DFTD2) in the calculation.)

Summary In summary, to understand the elementary steps of the reaction involved, we studied the molecular adsorption of CH3Cl/Cu(410) at 100 K, and its complete desorption at higher temperatures, 100 K < TD < 200 K. Temperature-programmed desorption (TPD) spectra show two CH3Cl desorption peaks. We attribute the low temperature TPD peak (TD ∼ 120 K) to CH3Cl desorbing from both step-edges and terraces, and the high temperature TPD peak (TD ∼ 160 K) to CH3Cl desorbing from the step-edges. Infrared reflectionabsorption spectra (IRAS) indicate that at low CH3Cl coverage (Θ = 0.06 ML), CH3Cl adsorbs with its molecular axis (Cl-C bond) aligned either parallel or perpendicular to [001]. At high CH3Cl coverage (Θ ≥ 0.09 ML), CH3Cl adsorbs with its molecular axis aligned perpendicular to [001].

Acknowledgement This work is supported in part by MEXT Grant-in-Aid for Scientific Research (JP17K06818, JP17H01057, JP15H05736, JP15KT0062, JP15K14147, JP26248006). Some of the numerical 17



calculations presented here were done using the computer facilities at the following institutes: CMC (Osaka University), ISSP, KEK, NIFS, and YITP. This work was also supported financially by Shin-Etsu Chemical Co., Ltd. Japan. J.S.G. acknowledges scholarship from MEXT. S.Z. acknowledges support from the NEDO Project "R&D Towards Realizing an Innovative Energy Saving Hydrogen Society based on Quantum Dynamics Applications", the Nikki-Saneyoshi Scholarship Foundation, and MEXT. We are grateful to Messrs. Tetsuya Inukai and Elvis Arguelles for their valuable discussions.

Supporting Information Available Determining the coverage Θ[ML] and fitting of the corresponding C-H symmetric stretching anti-absorption line shape for CH3Cl adsorbed on Cu(410).

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Cu(410): Interaction and Adsorption Geometry

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